The present invention is in the field of data communications, and more particularly, relates to the initialization and maintenance of synchronization of terminals in ring communications systems.
Ring communications networks generally include a plurality of terminals coupled in series along a unidirectional signal path. In such communications rings, any of the terminals coupled to the ring may transmit or receive digital signals on the ring according to a ring protocol. In the unusual cases where the terminals are all geographically close together, it is a relatively simple matter to synchronize all the terminals, for example, by providing the clock (or timing) signals for use at the respective terminals from a common oscillator. However, in a more typical ring network environment, there is no readily accesible master oscillator. To achieve inter-terminal synchronization in such systems, timing information is added to the transmitted data, and the terminals coupled to the ring are adapted to use this information to maintain synchronization. The incorporated reference discloses a particularly advantageous method for maintaining synchronization for a ring communications network where each terminal is adapted to transmit its data at its own associated data rate which may vary or drift independently from the data transmission rates of the other terminals coupled to the ring.
Such ring networks are effective in maintaining synchronization between terminals coupled to the ring, and permit the establishment of interterminal communications while that synchronization is maintained. However, those networks (particularly those using a phase locked loop for extracting timing information from incoming data) are vulnerable to system outages upon lack of synchronization (which generally occurs upon system power-up) or loss of synchronization (which might occur due to signal inactivity, i.e., no data level transitions in the incoming data for sufficiently long period for the phase locked loop to lose lock). Thereafter, to regain synchronization, and permit the re-establishment of communications links, a relatively long re-synchronization process must be accomplished.